Taking moments about the axis.
\[30 \times 4=20 \times 3+5 \times x\]
\[120=60+5x \rightarrow x=\frac{120-60}{5}=12m\]
Rotational Equilibrium
If an object is in rotational equilibrium, the resultant turning force or torque, equal to force times distance from an axis for each force, is zero about any given axis. This is known as the principle of moments. The horizontal bar below is in equilibrium. About the axis shown there is no net turning force - no net moment.
The sum of the clockwise moments equals the sum of the anticlockwise moments.
Taking moments about the axis.
Taking moments about the axis.